US5317053A - Method for production of high solid aqueous calcium carbonate suspensions - Google Patents

Method for production of high solid aqueous calcium carbonate suspensions Download PDF

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Publication number
US5317053A
US5317053A US07/924,428 US92442892A US5317053A US 5317053 A US5317053 A US 5317053A US 92442892 A US92442892 A US 92442892A US 5317053 A US5317053 A US 5317053A
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Prior art keywords
calcium carbonate
neutralized
accordance
weight
dispersant
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Expired - Fee Related
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US07/924,428
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English (en)
Inventor
Alan J. Brown
Michael S. Darsillo
O. Wayne Bentley, Jr.
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Imerys Minerals USA Inc
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ECC International Inc
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Priority to US07/924,428 priority Critical patent/US5317053A/en
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    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09D—COATING COMPOSITIONS, e.g. PAINTS, VARNISHES OR LACQUERS; FILLING PASTES; CHEMICAL PAINT OR INK REMOVERS; INKS; CORRECTING FLUIDS; WOODSTAINS; PASTES OR SOLIDS FOR COLOURING OR PRINTING; USE OF MATERIALS THEREFOR
    • C09D17/00—Pigment pastes, e.g. for mixing in paints
    • C09D17/004—Pigment pastes, e.g. for mixing in paints containing an inorganic pigment
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08K—Use of inorganic or non-macromolecular organic substances as compounding ingredients
    • C08K3/00—Use of inorganic substances as compounding ingredients
    • C08K3/18—Oxygen-containing compounds, e.g. metal carbonyls
    • C08K3/24—Acids; Salts thereof
    • C08K3/26—Carbonates; Bicarbonates
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K23/00—Use of substances as emulsifying, wetting, dispersing, or foam-producing agents
    • C09K23/10—Derivatives of low-molecular-weight sulfocarboxylic acids or sulfopolycarboxylic acids

Definitions

  • This invention relates generally to particulate calcium carbonates, and more specifically relates to the production of stable high solids aqueous slurries of such materials, for use, for example, in paper coating and other applications.
  • such high solids slurries even if initially displaying satisfactory viscosity, show an increase or buildup in viscosity as a function of time (i.e. with aging). In many instances such slurries will actually set after a relatively brief period, sometimes measured in hours to days, into a unusable gelled mass.
  • Prior art dispersants commonly used with calcium carbonate slurries include various types of water soluble polymeric dispersants. Among the most common and relatively effective agents used for these purposes, are the Dispex® products of Allied Colloids of Great Britain. In general, these constitute water soluble salts of polyacrylic or polymethacrylic acid and/or water soluble copolymers or water soluble copolymer derivatives of the type described e.g. in British Patent Specification No. 1,414,964.
  • dispersants have been introduced which are superior to the aforementioned products, particularly as regards longer term stability. These dispersants are produced by co-polymerization of a sulfonic acid containing monomer with acrylic acid which tends to produce a dispersant that adds charge stabilization to the steric stabilization that a pure polyacrylic acid type dispersant offers. Examples of these sulfonated copolymers are the Allied Colloids products DP 2020 and DP 2157. See European Patent Publications Nos. 0108842 and 0129329, and U.S. Pat. No. 4,509,987. These polymer products are fully neutralized with sodium cation.
  • dispersing agents are water soluble carboxyl-containing polymers which are converted to the salt form by at least one salt-forming agent which has a polyvalent function.
  • the cation used may comprise calcium, magnesium, zinc, copper, lead, aluminum, and chromium.
  • carboxyl-containing polymers are described, including those derived from (meth)acrylic acid, itaconic acid, crotonic acid, etc.
  • the acid sites of the dispersing agent which are not neutralized by the polyvalent cation can be neutralized by a salt-forming agent having a monovalent function to render the dispersant fully neutralized.
  • U.S. Pat. No. 4,840,985 to Gonnet et al discloses a process for preparing a stable aqueous mineral suspension of calcium carbonate using a grinding agent which may be a homopolymer or copolymers which are predominantly acrylic acid and contain at least one monomer selected from the group consisting of (meth)acrylic acid, etc., wherein from 40 to 80% of the acid groups of the homopolymer and copolymers have been neutralized with neutralizing agent, at least one of which has a monovalent function. Thus, the polymer dispersants are partially neutralized.
  • a grinding agent which may be a homopolymer or copolymers which are predominantly acrylic acid and contain at least one monomer selected from the group consisting of (meth)acrylic acid, etc., wherein from 40 to 80% of the acid groups of the homopolymer and copolymers have been neutralized with neutralizing agent, at least one of which has a monovalent function.
  • neutralizing agent at least one of which has a mono
  • a yet further object of the invention is to provide new dispersing compositions which are highly useful in producing high solids slurries of mineral particles, such as a calcium carbonate slurry, which slurries are stable over very long periods of time.
  • a slurry is formed by dispersing in water at least 70% by weight of a finely divided mineral particulate, utilizing as a dispersing agent from 20 to 34 lbs/ton dry mineral (expressed on the basis of 40% actives), and preferably from 22 to 24 lbs/ton dry mineral (expressed on the basis of 40% actives), of a water soluble copolymer of acrylic acid and a sulfonated vinyl monomer, which polymer is converted to the salt form by from about 30 to 50% by weight of its carboxylic acid sites being neutralized by a polyvalent cation, with substantially the balance of the said carboxylic acid sites being neutralized with a monovalent cation.
  • the carboxylic acid sites of the resulting copolymer are therefore fully neutralized.
  • the polyvalent cation can be calcium, aluminum or magnesium, and preferably is calcium. Preferably from about 35 to 45% of the carboxylic acid sites are neutralized with the polyvalent cation.
  • the monovalent cation is preferably sodium; but other monovalent cations can be utilized. To achieve these purposes, such compounds as calcium hydroxide and sodium hydroxide may be used.
  • the dispersed material will have a PSD (particle size distribution) such that at least 90% by weight of the particles have an ESD (equivalent spherical diameter) of less than 2 ⁇ m.
  • PSD particle size distribution
  • ESD equivalent spherical diameter
  • the slurry of such a particulate can be formed by wet media grinding a relatively coarse feed calcium carbonate in the presence of the dispersant; the grinding can be accomplished at a wide range of solids, e.g. as low as 15%, with the solids being adjusted by filtration to the desired final product requirements.
  • the sulfonated dispersing agent used in this invention is prepared from a starting sulfonated copolymer of the type mentioned above, which more specifically comprises a copolymer of from 1 to 75% (preferably 5 to 10%) by weight AMPS (2-acrylamido 2-methyl propane sulfonic acid) and 99 to 25% by weight acrylic acid, having a molecular weight in the range 1,000 to 20,000 (preferably 2,000 to 6,000).
  • Dispersant polymers of this type are more fully described in European Patent Publications Nos. 0.129329, 0.108842, and U.S. Pat. No. 4,509,987. Also see Japanese Patent Publication 160062/80 (Application 54274/79).
  • Such an unneutralized acidic starting polymer in accordance with the present invention, is partially neutralized with monovalent cations, such that 50 to 70% of the carboxylic acid sites are neutralized. Thereafter, the remaining such carboxylic acid sites are neutralized with polyvalent cation.
  • a product of the invention will from time to time in this specification be referred to as a sulfonated mixed cation carboxylic acid group neutralized dispersant (or briefly an "SMC" dispersant).
  • a coarse natural calcium carbonate (e.g. one which has been classified to 60% ⁇ 2 ⁇ m) is wet sand ground at high solids, in the presence of the SMC dispersant, to 90% ⁇ 2 ⁇ m, after which the sand is removed by screening.
  • a filtration step may be used to achieve the final desired high solids slurry.
  • the SMC dispersant used in the media grinding is previously prepared by reacting the selected polyvalent and monovalent cation with the free acid sulfonated copolymer described above, in ratios appropriate to yield a carboxylic acid group neutralized copolymer, with 50 to 70% of the carboxyl groups being converted to salt form by monovalent cation, and the balance of the carboxyl groups being converted by polyvalent cation.
  • the dispersants of the invention can be used to effectively and stably disperse other fine particle sized mineral particulates in addition to calcium carbonates, including, for example, kaolins, talcs, gypsum and the like.
  • FIG. 1 is a graph of viscosity as a function of time for a high solids calcium carbonate slurry prepared according to the present invention, utilizing various dosage levels of an SMC dispersant;
  • FIG. 2 is a graph similar to FIG. 1, showing corresponding data for a further SMC dispersant
  • FIG. 3 is a graph similar to FIGS. 1 and 2, but depicting viscosities achieved in high solids calcium carbonate slurries for three different SMC dispersants as a function of time, with the dosage level being fixed;
  • FIGS. 4 and 5 are graphs similar to FIG. 3, but showing viscosities achieved where the products of the invention are used, as compared to use of prior art products;
  • FIG. 6 is a graph similar to FIG. 4, showing the viscosities achieved where the same series of products are evaluated for 21 day stability over a range of dosage levels.
  • SMC dispersants were prepared by dissolving the unneutralized acidic sulfonated copolymer in water and adding NaOH and Ca(OH) 2 in ratios to result in products which contain respectively 50, 60, and 70% carboxylic acid groups that are neutralized with sodium, with the remaining carboxylic acid groups neutralized with calcium. These will be referred to in the ensuing Examples as SMC 50/25, SMC 60/20, and SMC 70/15.
  • the starting polymer in this example was the sodium neutralized DP 2020 product of Allied Colloids of Great Britain, which was converted to the acid form by ion exchange. This copolymer contained 8% sulfonate groups (AMPS) and 92% acrylic groups in the copolymer backbone as determined by Carbon - 13 nuclear magnetic resonance studies.
  • AMPS 8% sulfonate groups
  • the SMC 50/25 dispersant of Example I was used in the preparation of a series of 75% solids calcium carbonate aqueous slurries in which the dosage rate was varied from 18 lbs/ton to 28 lbs/ton based on dry carbonate solids.
  • the dispersant dose levels include 40% active polymer and 60% water. All dosage levels in this specification are based on this ratio.
  • the slurries were prepared by sand media grinding of a 60% ⁇ 2 ⁇ m calcium carbonate to 90% ⁇ 2 ⁇ m in the presence of the SMC dispersant.
  • the resultant screened (to remove sand) slurries were evaluated for slurry stability over a 21 day period.
  • Example II The SMC 60/20 dispersant of Example I was used in the preparation of a further series of 75% solids calcium carbonate aqueous slurries as described in Example II. The resulting data is shown in FIG. 2 which plots viscosity as in Example I. It will be seen that in this instance a highly time-stable slurry was obtained for dosages as low as 20 lbs/ton. In fact, highly stable slurries were obtained for the four tests shown at 20 lbs/ton, 22 lbs/ton, 24 lbs/ton, and 28 lbs/ton. Only the 18 lbs/ton dosage rate yielded viscosities outside the desirable range, i.e. less than 20,000 CPS. The SMC 60/20 dispersant is considered relatively optimal for use in the present invention.
  • Example 3 the three dispersants prepared in Example I were used in preparation of a series of 75% solids calcium carbonate aqueous slurries using in each instance a dosage of 22 lbs/ton.
  • the Helipath viscosity in CPS was evaluated for each of the slurries as a function of aging. The results are shown in FIG. 3. It will be seen that only the SMC 60/20 in this instance yielded viscosities in the desired range, in fact an outstandingly stable product over the entire 21 day period, with virtually no increase in viscosity.
  • Dispersant No. 1 is the Dispex N40 product of Allied Colloids, of Great Britain, being the conventional water soluble salts of polyacrylic or polymethacrylic acid or derivatives, of the type described e.g. in British Patent Specification No. 1,414,964.
  • Dispersant 2 is an improved product of Allied Colloids understood to be of the type described in European Patent Publication No. 0129329 (discussed previously herein).
  • Dispersant 3 is a product of Coatex S.A. of Caluire, France, believed to be of the type described in U.S. Pat. No. 4,775,420, previously discussed herein.
  • Dispersant No. 4 is a fully neutralized sulfonated polymer (i.e. neutralized with sodium cation only) as described in U.S. Pat. No. 4,509,987.
  • Dispersant No. 5 is the SMC 60/20 product of the present invention, previously referred to. The results of these tests are shown in FIGS. 4 and 5.
  • Dispersants 1 and 3 yield results that are completely unacceptable over the entire period of evaluation.
  • Dispersants 2, 4 and 5 are all found to yield acceptable results up to 14 days.
  • dispersant 5 is found to yield an entirely acceptable, indeed remarkably outstanding result, over the entire period extending up to 21 days.
  • Example V the same five dispersants described in Example V were evaluated over a range of dosage levels where each of the said products was used to prepare 75% solids calcium carbonate slurries as described in the previous Examples. The evaluatory period was again 21 days. The results of these tests are shown in FIG. 6, from which it will first be seen that the product of the invention, i.e. dispersant 5 yields a virtually flat curve having fully acceptable viscosity stabilities for all dosages from 20 up to 28 lbs/ton. In comparison, the various other dispersants depicted either are not acceptable over the entire range of dosage levels; or the dose level required to obtain stability (where acceptable) is generally higher than that required with the dispersant of the invention. Thus, the striking advantages of the invention, as illustrated in FIG.

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  • Chemical & Material Sciences (AREA)
  • Organic Chemistry (AREA)
  • Materials Engineering (AREA)
  • Engineering & Computer Science (AREA)
  • Polymers & Plastics (AREA)
  • Health & Medical Sciences (AREA)
  • Medicinal Chemistry (AREA)
  • Chemical Kinetics & Catalysis (AREA)
  • Inorganic Chemistry (AREA)
  • Life Sciences & Earth Sciences (AREA)
  • Wood Science & Technology (AREA)
  • Compounds Of Alkaline-Earth Elements, Aluminum Or Rare-Earth Metals (AREA)
  • Emulsifying, Dispersing, Foam-Producing Or Wetting Agents (AREA)
  • Colloid Chemistry (AREA)
  • Compositions Of Macromolecular Compounds (AREA)
  • Paper (AREA)
  • Nitrogen And Oxygen Or Sulfur-Condensed Heterocyclic Ring Systems (AREA)
  • Steroid Compounds (AREA)
US07/924,428 1990-06-29 1992-07-31 Method for production of high solid aqueous calcium carbonate suspensions Expired - Fee Related US5317053A (en)

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EP (1) EP0548086B1 (enrdf_load_stackoverflow)
JP (1) JPH06504939A (enrdf_load_stackoverflow)
KR (1) KR0171906B1 (enrdf_load_stackoverflow)
AT (1) ATE143681T1 (enrdf_load_stackoverflow)
AU (1) AU655429B2 (enrdf_load_stackoverflow)
BR (1) BR9106599A (enrdf_load_stackoverflow)
CA (1) CA2086231A1 (enrdf_load_stackoverflow)
DE (1) DE69122513T2 (enrdf_load_stackoverflow)
DK (1) DK0548086T3 (enrdf_load_stackoverflow)
ES (1) ES2091932T3 (enrdf_load_stackoverflow)
FI (1) FI100979B (enrdf_load_stackoverflow)
NO (1) NO300228B1 (enrdf_load_stackoverflow)
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Cited By (12)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5653795A (en) * 1995-11-16 1997-08-05 Columbia River Carbonates Bulking and opacifying fillers for cellulosic products
US5676747A (en) * 1995-12-29 1997-10-14 Columbia River Carbonates Calcium carbonate pigments for coating paper and paper board
US5676748A (en) * 1995-12-29 1997-10-14 Columbia River Carbonates Bulking and opacifying fillers for paper and paper board
US5676746A (en) * 1995-04-11 1997-10-14 Columbia River Carbonates Agglomerates for use in making cellulosic products
US5913973A (en) * 1997-07-08 1999-06-22 Ecc International Inc. Method for producing high solids aqueous acid-resistant calcium carbonate suspensions and product thereof
US20030202929A1 (en) * 2002-04-25 2003-10-30 Golley Christopher R. L. Stabilization of the viscosity of formamidine sulfinic acid bleached calcium carbonate slurries
US7033428B2 (en) * 1996-12-30 2006-04-25 Minerals Technologies Inc. Acid stabilized calcium carbonate an method of making it
US20100071863A1 (en) * 2008-09-22 2010-03-25 Hercules Inc. Copolymer blend compositions for use to increase paper filler content
US20100105832A1 (en) * 2008-10-29 2010-04-29 Basf Se Process for preparing aqueous polyacrylic acid solutions by means of controlled free-radical polymerization
WO2011054789A1 (de) 2009-11-04 2011-05-12 Basf Se Verfahren zur herstellung von wässrigen polyacrylsäurelösungen
US20140165877A1 (en) * 2011-04-01 2014-06-19 Patrick A.C. Gane Self-binding pigment hybrid
US10889661B2 (en) 2010-12-17 2021-01-12 Basf Se Stable polyacrylic acids, their manufacture and their use

Families Citing this family (8)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
DE4225922A1 (de) * 1992-08-05 1994-02-10 Bayer Ag Verwendung von Salzen mehrbasiger organischer Säuren mit mehrwertigen Kationen als Dispergiermittel für wäßrige Dispersionen und wäßriger Dispersionen, die derartige Dispergiermittel enthalten
US6150851A (en) * 1997-06-06 2000-11-21 Tadahiro Ohmi Charge transfer amplifier circuit, voltage comparator, and sense amplifier
FR2818166B1 (fr) * 2000-12-20 2003-10-31 Coatex Sas Agent d'aide au broyage et/ou de dispersion de materiaux mineraux en suspension aqueuse. suspensions aqueuses obtenues et leurs utilisations
FI20096408L (fi) * 2009-12-30 2011-07-01 Kemira Oyj Dispersioainekoostumus, sen käyttö ja sitä käsittävä päällystyskoostumus ja menetelmä päällystyskoostumuksen tai lietteen viskositeettiominaisuuksien parantamiseksi
EP2390284B2 (en) 2010-05-28 2017-03-15 Omya International AG Process for manufacturing high solids suspensions of mineral materials
GB201407798D0 (en) * 2014-05-02 2014-06-18 Imerys Minerals Ltd Dispersant
WO2018081964A1 (en) 2016-11-02 2018-05-11 Basf Se Acrylic acid polymers neutralized with sodium and calcium ions and having a narrow molecular weight distribution
CN107556407A (zh) * 2017-09-18 2018-01-09 江苏亘德科技有限公司 一种悬浮聚合无机分散剂的制备方法

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EP0108842A1 (en) * 1982-11-10 1984-05-23 Ciba Specialty Chemicals Water Treatments Limited Calcium carbonate dispersions
US4775420A (en) * 1986-08-22 1988-10-04 Coatex, S.A. Complex pigment compositions for coating of paper
US4840985A (en) * 1982-08-06 1989-06-20 Coatex, S. A. Grinding agent based and/or copolymers for aqueous suspension of coarse mineral materials for pigment applications
EP0129329B1 (en) * 1983-05-20 1989-08-02 Ciba Specialty Chemicals Water Treatments Limited Water soluble polymers and dispersions containing them
US4868228A (en) * 1982-08-06 1989-09-19 Coatex, S.A. Grinding agent based upon neutralized acrylic polymers and/or copolymers for aqueous suspensions of coarse mineral materials for pigments applications
US4892902A (en) * 1986-11-07 1990-01-09 Nippon Shokubai Kagaku, Co., Ltd. Method for production of aqueous dispersion of inorganic pigment

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DE4004953C3 (de) * 1990-02-19 1998-01-29 Stockhausen Chem Fab Gmbh Verfahren zur Herstellung von neutralisierten Polyacrylsäuren und ihre Verwendung als Mahl- und Dispergierhilfsmittel

Patent Citations (7)

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US4840985A (en) * 1982-08-06 1989-06-20 Coatex, S. A. Grinding agent based and/or copolymers for aqueous suspension of coarse mineral materials for pigment applications
US4868228A (en) * 1982-08-06 1989-09-19 Coatex, S.A. Grinding agent based upon neutralized acrylic polymers and/or copolymers for aqueous suspensions of coarse mineral materials for pigments applications
EP0108842A1 (en) * 1982-11-10 1984-05-23 Ciba Specialty Chemicals Water Treatments Limited Calcium carbonate dispersions
US4509987A (en) * 1982-11-10 1985-04-09 Allied Colloids Limited Calcium carbonate dispersions
EP0129329B1 (en) * 1983-05-20 1989-08-02 Ciba Specialty Chemicals Water Treatments Limited Water soluble polymers and dispersions containing them
US4775420A (en) * 1986-08-22 1988-10-04 Coatex, S.A. Complex pigment compositions for coating of paper
US4892902A (en) * 1986-11-07 1990-01-09 Nippon Shokubai Kagaku, Co., Ltd. Method for production of aqueous dispersion of inorganic pigment

Cited By (16)

* Cited by examiner, † Cited by third party
Publication number Priority date Publication date Assignee Title
US5676746A (en) * 1995-04-11 1997-10-14 Columbia River Carbonates Agglomerates for use in making cellulosic products
US5653795A (en) * 1995-11-16 1997-08-05 Columbia River Carbonates Bulking and opacifying fillers for cellulosic products
US5676747A (en) * 1995-12-29 1997-10-14 Columbia River Carbonates Calcium carbonate pigments for coating paper and paper board
US5676748A (en) * 1995-12-29 1997-10-14 Columbia River Carbonates Bulking and opacifying fillers for paper and paper board
US7033428B2 (en) * 1996-12-30 2006-04-25 Minerals Technologies Inc. Acid stabilized calcium carbonate an method of making it
US5913973A (en) * 1997-07-08 1999-06-22 Ecc International Inc. Method for producing high solids aqueous acid-resistant calcium carbonate suspensions and product thereof
US20030202929A1 (en) * 2002-04-25 2003-10-30 Golley Christopher R. L. Stabilization of the viscosity of formamidine sulfinic acid bleached calcium carbonate slurries
US7105139B2 (en) 2002-04-25 2006-09-12 Imerys Pigments, Inc. Stabilization of the viscosity of formamidine sulfinic acid bleached calcium carbonate slurries
US20100071863A1 (en) * 2008-09-22 2010-03-25 Hercules Inc. Copolymer blend compositions for use to increase paper filler content
US8163134B2 (en) * 2008-09-22 2012-04-24 Hercules Incorporated Copolymer blend compositions for use to increase paper filler content
US20100105832A1 (en) * 2008-10-29 2010-04-29 Basf Se Process for preparing aqueous polyacrylic acid solutions by means of controlled free-radical polymerization
EP2182011A1 (de) 2008-10-29 2010-05-05 Basf Se Verfahren zur Herstellung von wässrigen Polyacrylsäurelösungen mittels kontrollierter radikalischer Polymerisation
WO2011054789A1 (de) 2009-11-04 2011-05-12 Basf Se Verfahren zur herstellung von wässrigen polyacrylsäurelösungen
US9056926B2 (en) 2009-11-04 2015-06-16 Basf Se Process for preparing aqueous polyacrylic acid solutions
US10889661B2 (en) 2010-12-17 2021-01-12 Basf Se Stable polyacrylic acids, their manufacture and their use
US20140165877A1 (en) * 2011-04-01 2014-06-19 Patrick A.C. Gane Self-binding pigment hybrid

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NO925011L (no) 1993-02-22
ATE143681T1 (de) 1996-10-15
FI925916L (fi) 1992-12-29
NO925011D0 (no) 1992-12-23
DE69122513D1 (de) 1996-11-07
KR930701523A (ko) 1993-06-12
EP0548086A4 (en) 1994-03-16
AU655429B2 (en) 1994-12-22
DE69122513T2 (de) 1997-02-06
DK0548086T3 (enrdf_load_stackoverflow) 1997-03-10
AU8210991A (en) 1992-01-23
ES2091932T3 (es) 1996-11-16
BR9106599A (pt) 1993-06-01
EP0548086B1 (en) 1996-10-02
WO1992000347A1 (en) 1992-01-09
JPH06504939A (ja) 1994-06-09
FI925916A0 (fi) 1992-12-29
FI100979B (fi) 1998-03-31
EP0548086A1 (en) 1993-06-30
NO300228B1 (no) 1997-04-28
CA2086231A1 (en) 1991-12-30
KR0171906B1 (ko) 1999-03-30

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